High strength cold rolled steel sheet excellent in weldability and method for manufacturing the same

ABSTRACT

A high strength cold rolled steel sheet excellent in weldability and has a TS of 440 MPa or more includes a composition including C: 0.0005 to 0.005%, Si: 0.1 to 1.0%, Mn: 1 to 2.5%, P: 0.01 to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N: 0.007% or less, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to 0.5%, and Ni: 0.03 to 0.5% by mass with the balance Fe and incidental impurities; and a microstructure constituted by a ferrite single phase.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2009/063622, withan international filing date of Jul. 24, 2009 (WO 2010/016430 A1,published Feb. 11, 2010), which is based on Japanese Patent ApplicationNo. 2008-201735, filed Aug. 5, 2008, the subject matter of which isincorporated by reference.

TECHNICAL FIELD

This disclosure relates to a high strength cold rolled steel sheet withformability and weldability that is suitably used for structures such asrailway vehicles, automobiles, and ships. In particular, the disclosurerelates to a high strength cold rolled steel sheet having a tensilestrength TS of 440 MPa or more and a method for manufacturing the same.

BACKGROUND

With the progress of degassing technologies in steelmaking processes, ithas been possible to manufacture a large amount of ultra low carbonsteel whose amount of C is reduced to 0.0030% or less by mass atrelatively low cost. An interstitial free (IF) cold rolled steel sheetwith excellent formability made by adding a carbonitride-forming elementsuch as Ti or Nb to ultra low carbon steel has been widely used forautomobile components, electric device components, and the like. Thus,various IF cold rolled steel sheets have been developed. For example,Japanese Unexamined Patent Application Publication Nos. 61-246344 and1-149943 disclose cold rolled steel sheets with excellent formability.In those cold rolled steel sheets, resistance to cold-work embrittlementis improved by further adding B to the IF steel to which Ti or Nb isadded. Japanese Unexamined Patent Application Publication No. 2-232342discloses a deep-drawing steel sheet with excellent brazing propertiesobtained by further adding Ni to the IF steel to which Ti or Nb isadded.

In recent years, an automotive steel sheet having higher strength hasbeen developed in terms of weight reduction of a car body and crashsafety of automobiles. Furthermore, a tailor-welded-blank made bycombining two or more steel sheets having different thicknesses andcharacteristics through welding has been used as an automotive steelsheet to reduce the numbers of steps and dies. Therefore, there has beena growing demand for a high strength steel sheet with excellentformability and weldability, particularly a high strength cold rolledsteel sheet having a TS of 440 MPa or more.

The IF cold rolled steel sheet described above is desired inconsideration of formability. However, the weldability of atailor-welded-blank that uses an IF cold rolled steel sheet has hardlybeen investigated. Regarding the weldability of a tailor-welded-blank,Japanese Unexamined Patent Application Publication No. 2003-94170discloses a method for manufacturing a tailor-welded-blank by weldingsteel sheets having different thicknesses through plasma welding that isperformed with low equipment cost, at high speed, and without using awelding metal. In that method, a weld defect called “humping bead” isprevented by adjusting the amount of C of the thicker steel sheet to0.1% or more by mass or by adjusting the amount of Si to 0.8% or more bymass.

However, in the method for manufacturing a tailor-welded-blank describedin JP '170, the amount of C of at least one of the steel sheets needs tobe adjusted to 0.1% or more by mass or the amount of Si needs to beadjusted to 0.8% or more by mass. This poses a problem in that theformability of the tailor-welded-blank is significantly deteriorated.

In addition, high speed plasma arc welding easily causes the formationof humping beads. This poses a problem of high speed welding, that is,difficulty in improving productivity. It could therefore be helpful toachieve high speed welding by improving a steel sheet.

In other words, it could be helpful to provide a high strength coldrolled steel sheet with excellent weldability in which humping beads arenot formed by performing plasma welding at high speed and that has a TSof 440 MPa or more, which does not deteriorate the formability of atailor-welded-blank. It could also be helpful to provide a method formanufacturing the high strength cold rolled steel sheet.

SUMMARY

We discovered the following after investigating the formability andweldability of a high strength cold rolled steel sheet in which humpingbeads are not formed by performing plasma welding at high speed and thathas a TS of 440 MPa or more, which does not deteriorate the formabilityof a tailor-welded-blank:

-   -   i) The formation of humping beads during high speed plasma        welding can be suppressed by adding Cu to IF steel, and further        prevented by controlling the amount of O in the steel within a        proper range.    -   ii) Excellent formability of a tailor-welded-blank is achieved        by adjusting the amount of C to 0.005% or less by mass, by using        IF steel to which Ti is added, and by using a microstructure        constituted of a ferrite single phase.

Accordingly, we provide a high strength cold rolled steel sheet that isexcellent in weldability and has a TS of 440 MPa or more, including acomposition including C: 0.0005 to 0.005%, Si: 0.1 to 1.0%, Mn: 1 to2.5%, P: 0.01 to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N:0.007% or less, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to0.5%, and Ni: 0.03 to 0.5% by mass with the balance Fe and incidentalimpurities; and a microstructure constituted of a ferrite single phase.

In the high strength cold rolled steel sheet, the composition preferablyfurther includes O: 0.0025 to 0.0080% by mass or at least one of Se:0.0005 to 0.01% and Te: 0.0005 to 0.01% by mass.

The high strength cold rolled steel sheet can be manufactured by amethod including the steps of hot-rolling a slab having the compositiondescribed above, coiling at a coiling temperature of 680° C. or less,pickling, cold-rolling at a reduction ratio of 40% or more, andperforming recrystallization annealing at 700 to 850° C.

A high strength cold rolled steel sheet with excellent weldability inwhich humping beads are not formed by performing plasma welding at highspeed and that has a TS of 440 MPa or more, which does not deterioratethe formability of a tailor-welded-blank can be manufactured.Furthermore, the high strength cold rolled steel sheet with excellentformability is suitably used for not only automobile components, butalso electric device components and the like.

DETAILED DESCRIPTION

Our steel sheets and methods will now be described in detail. Herein, %denotes the amount of elements expressed as percent by mass unlessspecified.

1) Composition C: 0.0005 to 0.005%

When the C amount is less than 0.0005%, a heavy burden is placed ondecarbonization refining at a steelmaking stage, which increases thecosts due to, for example, vacuum degassing. When the amount of C ismore than 0.005%, formability deteriorates. Thus, the amount of C is inthe range of 0.0005 to 0.005%, and is preferably 0.003% or less.

Si: 0.1 to 1.0%

Si is an element that is effective for imparting higher strength tosteel. To achieve such an effect, the amount of Si needs to be 0.1% ormore. However, an amount of Si of more than 1.0% causes embrittlement offerrite, which impairs the strength-ductility balance. Thus, the amountof Si is in the range of 0.1 to 1.0%, and is preferably 0.7% or less.

Mn: 1 to 2.5%

Mn is an element that is effective for imparting higher strength tosteel. To achieve such an effect, the amount of Mn needs to be 1% ormore. However, an amount of Mn of more than 2.5% facilitates centerlinesegregation in a slab and deteriorates the formability of end products.Thus, the amount of Mn is in the range of 1 to 2.5%. To prevent hotbrittleness due to FeS formation, Mn is combined with solid solution Sin the steel to form MnS. In this case, assuming that the amount of Mnis [Mn] and the amount of S is [S], it is preferable to satisfy([Mn]/55)/([S]/32)>100.

P: 0.01 to 0.2%

P is an element that is effective for imparting higher strength tosteel. To achieve such an effect, the amount of P needs to be 0.01% ormore. However, an amount of P of more than 0.2% not only may cause grainboundary fracture in an HAZ or deteriorate low temperature toughness ofa base metal or a welded portion, but also deteriorates an anti-crashproperty due to grain boundary segregation. Thus, the amount of P is inthe range of 0.01 to 0.2%.

S: 0.015% or Less

An amount of S of more than 0.015% deteriorates low temperaturetoughness of a base metal or a welded portion as with P. Thus, theamount of S is 0.015% or less, and a smaller amount is preferable. Asdescribed above, it is preferable to satisfy ([Mn]/55)/([S]/32)>100.

Sol. Al: 0.05% or Less

Al is normally used as a deoxidizing element at a steelmaking stage.Since the amount of O is controlled within a specific range, the amountof sol. Al is 0.05% or less. An amount of sol. Al of more than 0.05% isnot preferable because formability deteriorates due to a large amount ofAl₂O₃ and inclusions may cause weld cracking. Thus, the amount of sol.Al is 0.05% or less.

N: 0.007% or Less

An amount of N of more than 0.007% deteriorates the formability andanti-aging property. Thus, the amount of N is 0.007% or less, and asmaller amount is preferable.

Ti: 0.01 to 0.1%

Ti improves the formability and anti-aging property by forming aprecipitate with C or N. To achieve such an effect, the amount of Tineeds to be 0.01% or more. However, an amount of Ti of more than 0.1%increases an alloy cost. Thus, the amount of Ti is in the range of 0.01to 0.1%. To effectively produce the effect of B described below,assuming that the amount of Ti is [Ti] and the amount of N is [N], it ispreferable to satisfy [N]−(14/48)[Ti]≦0.

B: 0.0005 to 0.0020%

B improves the resistance to cold-work embrittlement when B exists in asolid solution state. To achieve such an effect, the amount of B needsto be 0.0005% or more. However, an amount of B of more than 0.0020%facilitates weld cracking. Thus, the amount of B is in the range of0.0005 to 0.0020%.

Cu: 0.05 to 0.5%

Cu is an element that is effective for imparting higher strength withoutdeteriorating formability and preventing formation of humping beadsduring high speed plasma welding. In particular, the effects areincreased when Cu coexists with O controlled within the range describedbelow in the steel. To achieve such effects, the amount of Cu needs tobe 0.05% or more.

However, an amount of Cu of more than 0.5% saturates the effects andsignificantly deteriorates surface quality. Thus, the amount of Cu is inthe range of 0.05 to 0.5%. The reason why the formation of humping beadsduring high speed plasma welding can be prevented when Cu coexists withO is uncertain, but it is believed that the viscosity of the steelmelted during welding is optimized, which improves the flowability ofmolten steel.

Ni: 0.03 to 0.5%

The content of Cu described above easily deteriorates surface quality.To prevent it, an amount of Ni of 0.03% or more needs to be added.However, an amount of Ni of more than 0.5% saturates the effect. Thus,the amount of Ni is in the range of 0.03 to 0.5%. Assuming that theamount of Ni is [Ni] and the amount of Cu is [Cu], it is preferable tosatisfy 0.25×[Cu] [Ni]≦0.75×[Cu].

Although the balance is Fe and incidental impurities, O: 0.0025 to0.0080% or at least one of Se: 0.0005 to 0.01% and Te: 0.0005 to 0.01%is preferably further contained because of the following reasons.

O: 0.0025 to 0.0080%

As described above, the formation of humping beads during high speedplasma welding can be further suppressed when O coexists with Cu. It isbelieved that the viscosity and surface tension of the molten steelduring welding is further improved when O coexists with Cu. To achievesuch an effect, the amount of O in the steel needs to be 0.0025% ormore, preferably 0.0040% or more. However, an amount of O of more than0.0080% saturates the effect, increases the cost for treating a slabsurface due to a large number of blowholes of a continuous casting slab,and deteriorates the formability of the steel because of an increase inthe number of inclusions.

Se: 0.0005 to 0.01% and Te: 0.0005 to 0.01%

As with O, Se and Te improve the viscosity and surface tension of themolten steel during welding and prevent the formation of humping beadsduring high speed plasma welding when they coexist with Cu. To achievesuch effects, the amount of Se or Te needs to be 0.0005% or more.However, an amount of Se or Te of more than 0.01% saturates the effects.

2) Microstructure

In terms of formability, a microstructure constituted of a ferritesingle phase is required. The ferrite single phase herein may be eithera polygonal ferrite phase or a bainitic ferrite phase or a mixturethereof. To ensure a TS of 440 MPa and prevent the excessive softeningof a welded portion, the average grain diameter of the ferrite phase ispreferably 50 μm or less.

3) Manufacturing Conditions

The high strength cold rolled steel sheet can be manufactured by amethod including the steps of hot-rolling a slab having the compositiondescribed above, coiling at a coiling temperature of 680° C. or less,pickling, cold-rolling at a reduction ratio of 40% or more, andperforming recrystallization annealing at 700 to 850° C.

Coiling Temperature after Hot Rolling: 680° C. or Less

When the coiling temperature is more than 680° C., a chemical compoundof P and Fe, Ti, or the like is easily formed, which impedes thedevelopment of a {111} texture that is preferred for deep-drawingformability when cold-rolling and annealing performed later. Thus, thecoiling temperature is 680° C. or less, preferably 650° C. or less.

Reduction Ratio of Cold Rolling: 40% or More

In terms of formability, the reduction ratio is 40% or more. In terms ofimprovement in formability and, particularly, deep drawability, thereduction ratio is preferably 50% or more.

Recrystallization Annealing Temperature: 700 to 850° C.

The annealing temperature needs to be 700° C. or more forrecrystallization. However, when the annealing temperature exceeds 850°C., ferrite grains are coarsened, which decreases strength ordeteriorates surface quality. Thus, the recrystallization annealingtemperature is in the range of 700 to 850° C. To sufficiently performrecrystallization, a steel sheet is preferably held at 750° C. or morefor 30 seconds or longer.

Manufacturing conditions of typical methods can be applied to othermanufacturing conditions. In other words, steel is smelted in aconverter or an electric furnace to form a slab through continuouscasting. For hot rolling, the slab may be rolled after heat treatment,directly rolled without heat treatment, or rolled after short-time heattreatment. Hot rolling may be performed at a finishing temperature equalto or higher than the Ar₃ transformation temperature as with typicalprocedures. The recrystallization annealing can be performed by boxannealing or continuous annealing. After the annealing, skin passrolling may be performed, for example, for the purpose of the adjustmentof surface roughness and the planarization of a plate shape.Subsequently, surface treatments such as a chemical conversion treatmentand a plating treatment may be conducted.

Example 1

Each of steel Nos. 1 to 7 having an elemental composition of 0.002%C-0.2% Si-1.8% Mn-0.05% P-0.005% S-0.02% sol. Al-0.003% N-0.04%Ti-0.0010% B with Cu, O, and Se shown in Table 1 was smelted by vacuummelting, heated at 1200° C. for 1 hour, and then rough-rolled to make asheet bar having a thickness of 35 mm. The sheet bar was heated at 1250°C. for 1 hour and finish-rolled such that the finish rolling enteringtemperature was 900° C. after seven passes. Subsequently, heat treatmentcorresponding to coiling was performed at 580° C. for 1 hour to obtain ahot rolled steel sheet having a thickness of 4 mm. The hot rolled steelsheet was descaled through pickling and cold-rolled at a reduction ratioof 60% to obtain a cold rolled steel sheet having a thickness of 1.6 mm.Recrystallization annealing in which heating is conducted at 830° C. for180 sec and cooling is then conducted at a cooling rate of 10° C./secwas performed using a salt bath. After pickling was performed to removethe salt attached to the surface of the steel sheet, skin pass rollingwas conducted at an elongation percentage of 0.5%.

Steel sheets having the same composition were plasma-welded at a weldingspeed of 0.2 to 1.4 m/min under the following fixed conditions toinvestigate the “presence” or “absence” of humping beads: weldingcurrent: 60A, Ar gas flow rate for plasma: 0.6 L/min, Ar gas flow ratefor shield: 10 L/min, nozzle size: 2.0 mmφ, and nozzle-sample distance:3 mm.

Table 1 shows the results.

In the existing plasma welding, the maximum speed that can achievewelding without forming humping beads was about 0.2 to 0.4 m/min. Incontrast, humping beads were not formed at a high welding speed of 1m/min in the sample (steel No. 3) containing Cu and at a high weldingspeed of 1 m/min or more in the samples (steel Nos. 4 to 7) furthercontaining O and Se. Accordingly, our samples have high speed plasmaweldability.

TABLE 1 Cu Ni O Se Welding speed (m/min) and Humping Steel (% by (% by(% by (% by bead presence (x) absence (∘) No. mass) mass) mass) mass)0.2 0.4 0.6 0.8 1.0 1.2 1.4 1 0 0 <0.002 0 ∘ x 2 0.01 0.05 <0.002 0 ∘ ∘x 3 0.05 0.03 <0.002 0 ∘ ∘ ∘ ∘ ∘ x 4 0.05 0.03 0.0040 0 ∘ ∘ ∘ ∘ ∘ ∘ 50.1 0.05 0.0030 0 ∘ ∘ ∘ ∘ ∘ ∘ 6 0.1 0.05 0.0050 0 ∘ ∘ ∘ ∘ ∘ ∘ ∘ 7 0.10.05 0.0030 0.0010 ∘ ∘ ∘ ∘ ∘ ∘ ∘

Example 2

Each of steel Nos. A to F having compositions shown in Table 2 wassmelted to obtain a slab through continuous casting. The slab was heatedat 1200° C. and then finish-rolled at a finishing temperature of 900° C.The slab was coiled at a coiling temperature of 580° C. to obtain hotrolled steel sheets having thicknesses of 6 mm and 4 mm. The hot rolledsteel sheets were pickled and cold-rolled at a reduction ratio of 60% toobtain cold rolled steel sheets having thicknesses of 2.4 mm and 1.6 mm.Continuous annealing was performed at an annealing temperature of 830°C. and skin pass rolling was conducted at an elongation percentage of0.5%.

Steel sheets having the same composition were plasma-welded inaccordance with the combinations of sheet thickness shown in Table 3under the following fixed conditions to investigate the “presence” or“absence” of humping beads: welding current: 60A, Ar gas flow rate forplasma: 0.6 L/min, Ar gas flow rate for shield: 10 L/min, nozzle size:2.0 mmφ, nozzle-sample distance: 3 mm, and welding speed: 1 m/min.Furthermore, TS and total elongation El in a direction perpendicular tothe rolled direction of the obtained steel sheets and an average r valuewere measured using JIS 5 test pieces.

Table 3 shows the results. The steel sheets having compositions of ourexamples exhibit a TS of 440 MPa or more and are excellent informability. For the steel sheets, humping beads are not formed duringhigh speed plasma welding.

TABLE 2 Composition (% by mass) Steel sol. (Mn/55)/ No. C Si Mn P S Al NTi B Cu Ni O (S/32) Other Note A 0.0015 0.2 1.9 0.079 0.007 0.050 0.00160.037 0.001 0.09 0.05 <0.002 158 — within the scope of the disclosure B0.0025 0.2 2.0 0.075 0.007 0.050 0.0015 0.035 0.0015 0.12 0.05 0.005 166— within the scope of the disclosure C 0.0015 0.7 1.2 0.030 0.006 0.0300.0020 0.061 0.0015 0.06 0.035 0.003 116 Se: 0.005 within the scope ofthe disclosure D 0.0012 0.6 1.3 0.036 0.008 0.026 0.0007 0.032 0.0006 00.06 <0.002 95 — outside the scope of the disclosure E 0.0015 0.5 0.80.030 0.007 0.030 0.0020 0.06 0.0015 0.15 0.080 0.005 66 — outside thescope of the disclosure F 0.0015 0.1 0.3 0.075 0.003 0.050 0.0015 0.0350.0015 0.1 0.05 0.007 58 — outside the scope of the disclosure

TABLE 3 Combi- Aver- Humping nation age bead Steel of sheet TS El rpresence (x) No. thickness (MPa) (%) value absence (∘) Note A 1.6/1.6450 40 1.6 ∘ Example B 1.6/1.6 510 35 1.4 ∘ Example 2.4/1.6 510 ∘Example C 2.4/1.6 470 38 1.5 ∘ Example D 1.6/1.6 480 38 1.5 xComparative Example E 1.6/1.6 420 43 1.4 ∘ Comparative Example F 1.6/1.6285 60 1.8 ∘ Comparative Example TS, El, and r value are materialproperty values in 1.6 mmt

1. A high strength cold rolled steel sheet excellent in weldability andhas a tensile strength of 440 MPa or more, comprising a compositionincluding C: 0.0005 to 0.005%, Si: 0.1 to 1.0%, Mn: 1 to 2.5%, P: 0.01to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N: 0.007% or less,Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to 0.5%, and Ni: 0.03to 0.5% by mass with the balance Fe and incidental impurities; and amicrostructure constituted of a ferrite single phase.
 2. The highstrength cold rolled steel sheet according to claim 1, furthercomprising O: 0.0025 to 0.0080% by mass.
 3. The high strength coldrolled steel sheet according to claim 1, comprising at least one of Se:0.0005 to 0.01% and Te: 0.0005 to 0.01% by mass.
 4. A method formanufacturing a high strength cold rolled steel sheet excellent inweldability and has a tensile strength of 440 MPa or more, comprisinghot-rolling a slab having the composition of claim 1, coiling at acoiling temperature of 680° C. or less, pickling, cold-rolling at areduction ratio of 40% or more, and performing recrystallizationannealing at 700 to 850° C.
 5. The high strength cold rolled steel sheetaccording to claim 2, further comprising at least one of Se: 0.0005 to0.01% and Te: 0.0005 to 0.01% by mass.
 6. A method for manufacturing ahigh strength cold rolled steel sheet excellent in weldability and has atensile strength of 440 MPa or more, comprising hot-rolling a slabhaving the composition of claim 2, coiling at a coiling temperature of680° C. or less, pickling, cold-rolling at a reduction ratio of 40% ormore, and performing recrystallization annealing at 700 to 850° C.
 7. Amethod for manufacturing a high strength cold rolled steel sheetexcellent in weldability and has a tensile strength of 440 MPa or more,comprising hot-rolling a slab having the composition of claim 3, coilingat a coiling temperature of 680° C. or less, pickling, cold-rolling at areduction ratio of 40% or more, and performing recrystallizationannealing at 700 to 850° C.
 8. A method for manufacturing a highstrength cold rolled steel sheet excellent in weldability and has atensile strength of 440 MPa or more, comprising hot-rolling a slabhaving the composition of claim 5, coiling at a coiling temperature of680° C. or less, pickling, cold-rolling at a reduction ratio of 40% ormore, and performing recrystallization annealing at 700 to 850° C.